The appropriate ratio of acetic anhydride to salicylic acid to produce the maximum yield of acetylsalicylic acid aspirin in the preparation reaction is 1:1, based on their stoichiometry in the balanced chemical equation:C7H6O3 salicylic acid + C4H6O3 acetic anhydride C9H8O4 acetylsalicylic acid + C2H4O2 acetic acid However, in practice, a slight excess of acetic anhydride is often used to ensure complete reaction of the salicylic acid and maximize the yield of acetylsalicylic acid. This excess is typically around 10% more than the stoichiometric amount.